1. Field of the Invention
The present invention relates to a stop method for a fuel cell system which is adapted for cold start-up.
2. Description of the Related Art
In recent years, fuel cell powered vehicles have been proposed, each of which includes a fuel cell system as a driving source of the vehicle. As a type of fuel cell system for such vehicles, a fuel cell system is known which includes a predetermined number of cell units stacked one on the other, each of which includes an anode, a cathode, and an electrolyte membrane sandwiched therebetween. When hydrogen is supplied to the anode and air (oxygen) is supplied to the cathode, electrical power generation is performed via an electrochemical reaction of hydrogen and oxygen which is accompanied by producing water. Even though water is mainly formed at the cathode during an operation of the fuel cell system, a portion of the water at the cathode may move to the anode through the electrolyte membrane sandwiched between the cathode and the anode. In addition, the reaction gases (hydrogen and oxygen) are humidified in order to prevent the electrolyte membrane from becoming dry.
When the power generation of the fuel cell system is to be stopped, the above-mentioned formed water and humidifying water remain in gas flow paths of the fuel cell units; therefore, when the power generation is stopped while water remains therein, the remaining water may freeze at low temperature, and the frozen water (ice) may block supply and discharge of the reaction gases (hydrogen and air), which leads to degradation of start-up performance at low temperature.
To solve such a problem, Published Japanese Translation No. 2000-512068 of the PCT International Application discloses a cold start-up method in which a fuel cell system is warmed by making the fuel cell system supply electrical energy to an external electrical circuit. Moreover, in order to improve start-up performance at low temperature, this published document discloses a technology in which formed water remaining in gas flow paths of the fuel cell units is discharged using a nitrogen gas or the like when the operation of the fuel cell system is stopped so as to prevent freezing of water in the gas flow paths during stop periods of the fuel cell system.
In the aforementioned prior art technology, start-up performance at low temperature is improved by discharging water remaining in the gas flow paths of the fuel cell units; however, because a nitrogen gas is used for discharging water remaining in the gas flow paths of the fuel cell units, a tank, such as a dedicated nitrogen container, must be provided in the vehicle, which is a problem in view of limited installation capacity of a fuel cell powered vehicle.
Furthermore, another method has been proposed in which reaction gases (hydrogen and oxidizing agent such as air) are supplied to a fuel cell system during stoppage of the fuel cell system; however, increase in hydrogen consumption is a problem because a significant amount of reaction gas (i.e., hydrogen) that does not contribute to power generation must be supplied to an anode to discharge remaining water at the anode.